Coopwood was born on May 1, 1827, in Lawrence County, Alabama. He moved to Texas in 1846 and fought in a cavalry regiment in the Mexican–American War. In 1854 he moved to California, where he was admitted to the bar, practicing in Los Angeles. Following the killing of Sheriff James R. Barton by the Flores Daniel Gang, Coopwood led twenty-six El Monte men, as a division of the posse in the manhunt for the gang.